I completed this project shortly after writing the last log, but didn't bother writing anything about it at the time. The idea I got there of putting the boards flat against each other worked fine, and I didn't need any Kapton tape between them either. So now I've gone from having a Sonic 3 cartridge that won't save, to having a Sonic 3 cartridge with 16 cumbersomely-switchable memory banks. Plus, the new chip is rated for 10,000 times as many read/write cycles as the old one, so it shouldn't need replacement until long after I'm gone. Thanks Cypress for continuing to make compatible chips!